In general, devices for removing the tread of worn tires, often called buffers or raspers, are well known. Tire buffing is part of the tire retreading operation. The method of tire retreading described herein is commonly referred to as “cold-process retreading.”
Typically the tire casing selected for retreading is buffed to remove excess rubber to provide a substantially evenly-textured crown for receiving a pre-cured tread strip and to provide a predetermined tire casing profile. Tire casings usually include a belt package (a package of steel belts or cables) underlying the road-engaging surface (e.g., the original tread) of the tire. Prior to retreading, the casing must be buffed, generally to a predetermined characteristic crown radius corresponding to the upper contour of the belt package. The casing is buffed to leave only a predetermined thickness, e.g., 3/32 of an inch, of material remaining over the top belt. The shoulder of the casing is also buffed (trimmed) to eliminate or reduce voids or patterns in the shoulder created by the original tread, and to provide, typically, a relatively straight profile between the casing side walls and the crown. A worn casing from each of the various models and sizes of new tires has a characteristic tire casing profile of a particular crown width, crown buffing radius and shoulder trim angle which must be created as an initial step in the buffing process.
After being buffed, the tire casing may then be examined for injuries, which are skived and filled with a repair gum. After completion of the skiving process, the buffed surface may be sprayed with tire cement that provides a tacky surface for application of a suitable layer of bonding material, such as cushion gum. Conventionally, the cushion gum is a layer of uncured rubber material, which optionally includes a low temperature vulcanizing agent and accelerator. The cushion gum can be placed over the crown. In some retreading operations, the spray cement can be omitted.
Then a cured tread strip, typically of a width corresponding to the width of the crown of the casing is cut to the length corresponding to the casing circumference and is disposed over the casing crown. Alternatively, continuous replacement treads in the shape of a ring (i.e., ring treads) have also been used to retread the buffed casing. A roller pressing process, commonly referred to as stitching, is next performed on the assembly to force air from between the tread strip and casing.
After stitching the tire assembly, which comprises the tire casing, the cushion gum and the tread, the assembly can be placed within a flexible rubber envelope. An airtight seal can be created between the envelope and the bead of the tire casing. The entire envelope, with the tire assembly disposed therein, can be placed within a curing chamber and subjected to elevated pressure and temperature for a predetermined period of time. The combination of exposure to elevated pressure and temperature for a duration of time binds the cushion gum to both the tire casing and the new tire tread.
The tire buffing step of the retreading process can require the face of the casing, which receives the replacement tire tread, to be buffed to a predetermined crown radius within a relatively small tolerance. In addition, with the advent of new replacement tire tread designs, such as the contoured replacement tread disclosed in U.S. Pat. No. 5,277,727, issued Jan. 11, 1994, it can also be necessary to ensure that the shoulder radius is also buffed precisely to a predetermined arc, or radius.
In present retreading processes, it is important that the surface of the tire casing be carefully buffed about the shoulder areas of the tire to ensure that the tread layer width is approximately the same as the buffed surface of the casing. If the shoulder areas are not sufficiently buffed and trimmed, the tread edges may come loose and/or the cushion gum extending beyond the tread edges will not bond to the casing shoulder. Such problems can reduce the longevity of the retreaded tire and adversely impact the appearance of the retreaded tire.
The tire casing is buffed to remove tread material and to achieve a desired surface texture. Typically, it is desired for the crown and the shoulders of the tire casing to have different textures. The surface texture can be measured on a visual scale propounded by the Rubber Manufacturer's Association (RMA) with a numerical value between 1 and 6 with 1 being the finest and 6 being the roughest texture. Typically, it is desired for the crown of the tire casing to have a RMA number of between 3 and 4 and for the shoulder to have an RMA number of about 2. The shoulders of the tire casing typically are subjected to the largest stresses encountered by the tire during service. The finer texture on the shoulders promotes better adhesion of the tire tread to the casing at these high stresses. If the shoulders received too much texturing, crack propagation in these areas is more likely. In instances where the shoulder receives too little texturing, the bond between the tire casing and the tread is poorer.
The current state of the art in tire casing buffing allows for a multiplicity of tread removal passes, substantially in a sideways, pass after pass method. Known manual devices which require an operator to physically direct the buffing machine's removal direction and speed produce time periods between tread removal passes where the rate of tread rubber removal is less than optimum. Known automated devices in which the buffing machine's removal direction and speed is predetermined by an operator also produce time periods of less than optimum tread rubber removal.
Thus, there exists a need for a tire buffing machine which is easy to use and which improves tire buffing efficiency.